1. Field of the Invention
This invention relates to a color-developing agent for pressure-sensitive recording. More specifically, this invention relates to a color-developing agent which is superior in color-developing ability and light fastness.
2. Prior Art
Hitherto, many kinds of electron-accepting color-developing agents are well-known. The typical color-developing agents include inorganic solid acids such as acid clay, activated clay, attapulgite (described in U.S. Pat. No. 2,712,507); substituted phenols and diphenols (described in the Japanese Patent Publication No. 9309/1985); p-substituted phenol formaldehyde polymers (described in the Japanese Patent Publication No. 20144/1967); aromatic carboxylic acid metal salts (described in the Japanese Patent Publication Nos. 10856/1974 and 1327/1977); 2,2'-bisphenol sulfone compounds (described in the Japanese Patent Laid-Open No. 106313/1979); and so on.
These well-known color-developing agent has both advantages and disadvantages. For example, inorganic solid acids are as advantages inexpensive and excellent in color-developing speed, but they have disadvantages that a color-developing ability deteriorates in a storage owing to the absorption of gases and moisture in the air, and the developed images discolor or fade prominently upon exposure to sunlight, fluorescent light, etc. The substituted phenols provide insufficient color-developing ability and inferior image density.
P-substituted phenol-formaldhyde polymers (p-phenyl-phenolnovolak resin etc.) have superior color-developing ability, but have the disadvantage that the coating sheet undergoes yellow upon exposure to sun light or gases in the air in the storage. In the Japanese Patent Application No. 159540/1985 (the Japanese Patent Laid-Open No. 19486/1987), inventors have proposed a color-developing sheet comprising a polyvalent metal salt of carboxylated terpenphenol resin as a novel color-developing agent, in order to avoid both the yellowing in storage by light and gases and the discoloration and fading of the developed image by chemicals such as plasticizer.